The most commonly used gas-fired water heater is the storage type, generally comprising an assembly of a water tank, a main gas burner to provide heat to the tank, a standing pilot burner to initiate the main burner on demand, an air inlet adjacent the burner near the base of the jacket, an exhaust flue and a jacket to cover these components. Another type of gas-fired water heater is the instantaneous type which has a water flow path through a heat exchanger heated, again, by a main burner initiated from a pilot burner flame. For convenience, the following description is in terms of storage type water heaters but the present invention is not limited to this type. Thus, reference to "water container," "water containment and flow means," "means for storing or containing water" and similar such terms includes water tanks, reservoirs, bladders, bags and the like in gas-fired water heaters of the storage type and water flow paths such as pipes, tubes, conduits, heat exchangers and the like in gas-fired water heaters of the instantaneous type.
A particular difficulty with many locations for water heaters is that they are also used for storage of other equipment such as lawn mowers, trimmers, snow blowers and the like. It is a common procedure for such machinery to be refueled in such locations.
There have been a number of reported instances of spilled gasoline and associated fumes being accidently ignited. There are many available ignition sources, such as refrigerators, running engines, electric motors, electric light switches and the like. However, gas water heaters have sometimes been suspected because they often have a pilot flame.
Any vapors from spilt or escaping flammable liquid or gaseous substances in a space in which an ignition source is present, provides a potential for ignition. "Fumes," "extraneous gases" or "extraneous fumes" is sometimes hereinafter used to encompass gases, vapors or fumes generated by a wide variety of liquid volatile or semi-volatile substances such as gasoline, kerosine, turpentine, alcohols, insect repellent, weed killer, solvents and the like as well as non-liquid substances such as propane, methane, butane and the like. Many inter-related factors influence whether a particular fuel spillage does lead to ignition. These factors include, among other things, the quantity, the nature and physical properties of the particular type of spilt fuel. Also influential is whether air currents in the room, either natural or artificially created, are sufficient to accelerate the spread of fumes, both laterally and in height, from the spillage point to an ignition point yet not so strong as to ventilate such fumes harmlessly, that is, such that air to fuel ratio ranges capable of enabling ignition are not reached given all the surrounding circumstances.
One surrounding circumstance is the relative density of the fumes. When a spilt liquid fuel spreads on a floor, normal evaporation occurs and fumes from the liquid form a mixture with the surrounding air that may, at some time and at some locations, be within the range that will ignite. For example, that range for common gasoline vapor is between 3% and 8% gasoline with air, for butane between 1% and 10%. Such mixtures form and spread by a combination of processes including natural diffusion, forced convection due to air current draughts and by gravitationally affected upward displacement of molecules of one less dense gas or vapor by those of another more dense. Most common fuels stored in households are, as used, either gases with densities relatively close to that of air (eg. propane and butane) or liquids which form fumes having a density close to that of air, (eg. gasoline, which may contain butane and pentane among other components is very typical of such a liquid fuel).
In reconstructions of accidental ignition situations, and when gas water heaters are sometimes suspected and which involved spilt fuels typically used around households, it is reported that the spillage is sometimes at floor level and, it is reasoned, that it spreads outwardly from the spill at first close to floor level. Without appreciable forced mixing, the air/fuel mixture would tend to be at its most flammable levels close to floor level for a longer period before it would slowly diffuse towards the ceiling of the room space. The principal reason for this observation is that the density of fumes typically involved is not greatly dissimilar to that of air. Combined with the tendency of ignitable concentrations of the fumes being at or near floor level is the fact that many gas appliances often have their source of ignition at or near that level.
The present invention aims to substantially lower the probability of ignition in typical fuel spillage circumstances.